1. Field of the Invention
This invention relates to the field of rotation control systems and particularly to optical shaft angle encoders for providing information relating to the rotation of an object.
2. State of the Art
Early attempts to measure shaft rotation were accomplished in 1840 by the Wheatstone Dial Telegraph, which utilized a stepper motor to rotate a wheel engraved with the letters of the alphabet, a concept which evolved into the present daisy wheel printers.
As early as 1879, synchro designs had evolved which permitted measurement of shaft rotation with relatively high speed and accuracy. Synchros and resolvers are essentially rotating transformers having a construction similar to electrical motors. Because synchros and resolvers require an alternating current drive signal and additional electronics to convert phase shifts and their output to useful digital form, they have relatively limited applicability in modern, miniature control systems.
In order to develop encoders for converting angular or linear position to numbers utilizable by a computer, a number of approaches have been developed. Magnetic encoders have been built by applying a pattern to magnetic materials or by recording techniques.
Optical encoders have, during the last twenty years, developed into the most widely used encoding device. The optical encoder uses a precise pattern applied to a transparent material to interrupt light falling on an optical sensor. A two-inch diameter encoder can typically produce from 200 to 2,000 pulses per revolution. The light source utilized is commonly a light-emitting diode or a small tungsten lamp. The sensors are commonly phototransistors, silicon photodiodes or PIN diodes.
Incremental encoders used in motion control usually provide two output signals, designated hereafter A and B, phased in the same way as sine and cosine. When both of the signals are utilized, the direction of rotation of the encoder can be derived utilizing well-known circuitry. To obtain increased resolution, both rising and falling edges of both A and B signals are often used. If the A and B signals are exactly 90.degree. apart in phase and perfectly symmetrical, the resulting four pulses per cycle will be evenly spaced. If the phase separation between A and B is not 90.degree., then the spacing between the pulses will not be uniform. When the sensors are arranged circumferentially around a single data track, any slight eccentricity of the code disk motion will produce a periodic error in the phase between A and B. This problem can be greatly reduced by arranging the sensors along a radial line, as shown for example in U.S. Pat. No. 4,074,071, since both A and B signals are then affected equally by disk eccentricity. A modular encoder produced in this manner can maintain 90.degree. plus or minus 9.degree. phase between A and B over a broad temperature and frequency band.
Another use of the analog encoder signal is for velocity measurement. An analog signal proportional to velocity is commonly used to control motor speed in digital position control systems. This signal is often generated by a separate tachometer-generator attached to the motor shaft. The tachometer-generator adds cost, shaft inertia, length and potential service costs to the system. Thus, a convenient and economical method of using the velocity information present in the encoder analog signal has been found to be desirable. Integrated circuit motion control systems for daisy wheel printers have now been developed to provide encoder and tachometer information.
In the circuits for obtaining tachometer information from optical encoders, velocity signals are obtained by differentiating the analog encoder signal with a capacitor coupled feedback amplifier. When this arrangement is utilized with sinusoidal signals derived from the encoder, the ripple is about plus or minus 20%, even when the A and B signals are exactly phased. When the A and B signals have a phase error, a step change in tachometer output also occurs. The step changes in the tachometer output insert very high frequency components into the system, even at low angular velocities, which may excite oscillatory motion in the servo control system.